In many modern communication systems and in particular in mobile communication systems only limited transmission bandwidths are available for real-time audio transmissions, such as voice or music transmissions for example. In order to transmit as many audio or voice channels as possible in real time by way of a transmission link with limited bandwidth, as via a radio network for example, provision is therefore frequently made for compressing the audio signals to be transmitted by means of realtime-capable or quasi-realtime-capable audio coding methods.
With such audio coding methods the intention is generally to reduce the quantity of data to be transmitted and therefore the transmission rate as much as possible without having too great an adverse effect on the subjective audible impression or in the case of voice transmissions the comprehensibility.
Effective compression of audio signals is also an important aspect in relation to the storage or archiving of audio signals.
Coding methods, wherein an audio signal to be transmitted is aligned time frame by time frame with an audio signal synthesized by means of an audio synthesis filter by optimizing filter parameters, prove to be particularly effective. Such a procedure is frequently also referred to as analysis by synthesis. The audio synthesis filter is hereby excited by an excitation signal that is preferably likewise to be excited. Filtering is frequently also referred to as formant synthesis. The filter parameters used can for example be what are known as LPC coefficients (LPC: Linear Predictive Coding) and/or parameters which specify a spectral and/or temporal envelope of the audio signal. The optimized filter parameters and parameters specifying the excitation signal are then transmitted time frame by time frame to the receiver, to form a synthetic audio signal there by means of an audio synthesis filter provided on the receiver side, said synthetic audio signal being as similar as possible to the original audio signal in respect of the subjective audible impression.
Such an audio coding method is known from the ITU-T recommendation G.729. A realtime audio signal with a bandwidth of 4 kHz can be reduced to a transmission rate of 8 kbit/s by means of the audio coding method described there. According to the G.729 recommendation the excitation signal is generated by means of a so-called adaptive code book in conjunction with a so-called fixed code book. A plurality of predetermined excitation signal sequences are permanently stored in the fixed code book and can be retrieved using a code book index. In contrast already generated excitation signal sequences are stored in the adaptive code book. A respective sequence of the excitation signal is generated by mixing a sequence from the adaptive code book with a sequence from the fixed code book. To optimize the excitation signal, for each time frame both the fixed and adaptive code books are searched for excitation signal sequences, which allow the best possible alignment of the synthetic audio signal with the audio signal to be transmitted. Information relating to access to the sequences found to be optimal from the fixed and adaptive code books is finally transmitted to the receiver as parameters specifying the excitation signal. At the receiver these parameters are used to reconstruct an excitation signal by means of a fixed and adaptive code book of the receiver.
The search through two code books to be carried out according to the G.729 recommendation for each time frame in real time however requires a significant computation outlay, necessitating complex processors.
It is also currently intended to synthesize an audio signal to be transmitted with a higher bandwidth to improve the audible impression. The expansion G.729EV of the G.729 recommendation currently under discussion attempts to expand the audio bandwidth from 4 kHz to 8 kHz.
Such a bandwidth expansion of the synthesized audio signal can be achieved by constructing a suitable excitation signal of higher bandwidth, for example 8 kHz, from a narrowband excitation signal, for example with a bandwidth of 4 kHz, in order to excite the audio synthesis filter over a broad band. Different procedures for forming such a broadband excitation signal are currently under discussion in this context. According to such discussion the broadband excitation signal can be generated by squaring the narrowband excitation signal in the time domain or by generating an expansion band by displacing or mirroring the frequency spectrum of the narrowband excitation signal. However said procedures distort the spectrum of the excitation signal anharmonically and/or a significant, audible phase error is caused in the spectrum.